1. Field of the Invention
The present invention relates to a method of producing a substrate for an optical disc such as CD, LD, MD, MO, PD, and DVD.
2. Description of the Related Art
Conventionally, substrates for optical discs, for example, substrates for compact discs are molded at a melted-resin temperature of 330.degree. C. under such molding conditions that the mold temperature (i.e., the temperature of the water for controlling the temperature of a mold ) is 70 to 120.degree. C., the cooling time ranges from the filling of the resin to the taking-out thereof and the pressure dwell time (hereinafter referred to as "the cooling time") is 3.5 sec. or more (the molding cycle: about 5.5 sec. or more).
Meanwhile, along with an increase in the demand for optical discs of late, it is required that the molding cycle be shortened in order to improve the production capacity. As a means for shortening the substrate molding cycle, shortening the time in mechanical operations, such as the time of the opening and closing operation of the mold and the taking-out operation, and shortening the time in the molding process, such as the time of the filling/cooling of the resin, are conceivable. However, in order to shorten the time in mechanical operations, it is required to remodel the machines, leading to a great loss of time and an increase in cost. On the other hand, with respect to the shortening of the time in the molding process, there are specifications to be maintained in accordance with the standards of the play-back signal level, the error signal level, warp etc. (e.g., the below-mentioned Yellow Book standards in the case of compact discs). It is very difficult to shorten the molding time with complying with these specifications.